Preheat the oven to 160C/325F/Gas 3. Butter and line a 20cm/8in or 23cm/9in springform cake tin.
Put the cubed butter in a saucepan with the honey and sugar and melt them together slowly, stirring occasionally. Once dissolved, bring the mixture to a boil and simmer for one minute, turning the heat down as necessary to ensure the pan doesn't boil over. Set aside to cool for 20 minutes.
Beat the eggs together in a jug, then stir them into the cooled honey mixture. Sift the flour and baking powder into a large mixing bowl and make a well in the centre.
Pour the honey mixture into the flour and stir, gradually incorporating the flour. As soon as it is well combined, stir in the fennel seeds and four tablespoons of the chopped chestnuts.
Pour the cake batter into the prepared tin. Combine the remaining chestnuts with the pine nuts and flaky sea salt in a small bowl and sprinkle them over the top of the cake.
Bake for 45-60 minutes, or until the cake has risen and a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin for five minutes, then turn it out onto a wire rack.
Warm two tablespoons of honey in a small pan until runny, then brush it generously over the top of the cake. You can serve the cake warm with cream as a dessert, or leave it to cool completely, wrap in foil and store in an airtight tin, where it should keep for four days.